Today, plastics are used in a wide variety of applications. Virtually anything can be made from plastic, including eating and drinking utensils, toys, phones, appliances, packaging materials, sports equipment, and even clothing. Plastics are advantageous because they are typically lightweight, relatively durable, generally easy to mold and form, and are fairly inexpensive.
The full value of plastics, however, has yet to be realized for products and materials that require high strength and resiliency characteristics, such as building and construction materials, automobile frames, heavy machinery, and the like. Generally, underuse of plastics in these areas stems from the fact that while many plastics are relatively strong and durable, they typically do not exhibit the tremendous strength or rigidity characteristics required for these types of applications. For instance, traditional structural members, such as frames, supports, and I-beams, are generally constructed of wood, steel, concrete, or some other high-strength material. Automobile frames and parts are typically constructed of steel. Heavy machinery is almost always made of steel or some other metal.
All of these traditional materials, however, have inherent disadvantages. Wood, for example, may rot, split, warp, crack, or even be eaten by termites. Steel is extremely heavy, can be difficult to form and shape, and is very expensive. Concrete is not only heavy, but must be formed and shaped while in its liquid state, which naturally prevents numerous applications. Accordingly, high-strength plastic materials exhibiting the strength characteristics of wood or steel, without the drawbacks, would be greatly beneficial for use in construction, as well as other applications requiring high-strength materials.
One way the strength of plastics may be increased is by reinforcing the plastic with fillers, such as glass fibers or particles. When combined with glass or a similar filler material, the plastic becomes significantly stiffer and more durable. However, even with added strength, reinforced plastic lacks the strength and resiliency required for many applications, such as those described above. Additionally, the combining process is intricate and expensive, further prohibiting its widespread use.
Another way in which the strength of plastics may be increased is through “orientation”. Orientation (or “molecular orientation”) generally refers to the alignment of molecules within a particular piece of plastic material. As the molecules are aligned, the orientation and crystalline structure of the polymer chains within the piece of plastic increases. The degree of the orientation and crystallization dictates the strength of the plastic. Orientation is typically achieved by the heated stretching of a length of plastic material in a temperature range between the glass transition temperature (Tg) and the melt temperature (Tm). For instance, conventional drawing processes may be used to orient the plastic. Other conventional processes, such as “calendering” and compression roll drawing, may also be used to stretch and orient the material. Accordingly, as the plastic is oriented and stretched, the polymer chains align within the plastic, and the overall strength of the plastic increases correspondingly.
Because of the nature of the orientation process (heating, stretching, etc.), a thermoplastic is generally used. A thermoplastic (as compared to a thermoset or thermosetting plastic) is a plastic that melts when heated and hardens to a rigid state when cooled sufficiently, thermoplastics are remoldable and weldable when heat is added, whereas thermosetting plastics cannot be welded or remolded when heated, they simply burn instead. Accordingly, thermoplastics may be recycled and reused several times. Examples of thermoplastics include polyethylene terephthalate (PET), polypropylene, polybutylene terephthalate (PBT), and other like materials.
When the plastic is oriented, its cross-sectional area is greatly reduced. A conventional orientation process will stretch the polymer chains within the plastic to 3-5 times their normal length, resulting in a strength of 3-5 times that of pre-oriented plastic. However, this stretching results in a corresponding loss in cross-sectional area of 3-5 times the original area of the material. Thus, although the plastic becomes much stronger when oriented, it also becomes very thin. Existing machines and processes are incapable of orienting thick sheets of material to arrive at large cross-sectional areas because the machines simply cannot handle the immense stress and force required to stretch the material around rollers or pull it via some other mechanism. Thus, while a highly-oriented length of PET, for example, may exhibit a tensile strength similar to that of steel, a single sheet or layer of the material is essentially only useful for straps, ties, or other similar products, as opposed to thick, rigid members, because of its extremely thin cross-sectional area.
However, if several oriented sheets of plastic material are combined in some manner (without disturbing the molecular orientation of the sheets), then the combination retains the tensile strength and resiliency properties caused by orientation, but also adds the increased compression strength and rigidity characteristic of a thicker member. At present, there are a few known processes for combining oriented layers of thermoplastic material, such as rolling several sheets of material around a heated roller to melt the sheets together. However, none of the known processes are capable of combining thick layers (i.e. a thickness greater than or equal to that of typical film, which is in the range of about 0.002 inch to 0.015 inch). An inherent characteristic of oriented thermoplastic is that when it is reheated, it tends to shrink back to its original length. This shrinkage produces a large constriction force as the polymer chains attempt to contract. As layers become thicker, the force exerted during shrinkage becomes larger, to the point where the forces are unmanageable. Thus, conventional processes cannot adequately combine several thick layers of oriented plastic to form a rigid, high-strength member.
Additionally, even though conventional processes can combine very thin sheets (less than 0.010 inch thick), because the sheets are so thin, the entire sheet becomes melted during heating. This melting causes the polymer chains within the material to shrink and distort, which in turn causes the sheet to lose the strength and resiliency characteristics it gained during orientation. Thus, the combined material will not have the desired strength characteristics needed for certain applications. Further, because the sheets are so thin, it would require a great number of sheets to build enough thickness to produce a member of adequate thickness for structural or industrial use.
Therefore, there is a long-felt but unresolved need in the art for systems and methods of creating high-strength plastic material that is durable, relatively inexpensive, and easy to produce. Specifically, there is a need for a machine or process that combines thick layers of highly-oriented thermoplastics to form a rigid, unitary, high-strength member. Moreover, there is a need for a machine or process that combines thick layers of highly-oriented thermoplastics with thick layers of other types of filler material. Alternatively, there is a further need for a machine or process that is capable of orienting plastic materials of varying shapes with large cross-sectional areas.